on_const doesn't support parent_label

Author: mejrs

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -898,7 +898,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         main_obligation,
                         diag.long_ty_path(),
                     );
-                    let CustomDiagnostic { message, label, notes, parent_label } =
+                    let CustomDiagnostic { message, label, notes, parent_label: _ } =
                         command.eval(None, &format_args);
 
                     if let Some(message) = message {
@@ -910,9 +910,6 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     for note in notes {
                         diag.note(note);
                     }
-                    if let Some(parent_label) = parent_label {
-                        diag.span_label(impl_span, parent_label);
-                    }
                 } else if let Some(impl_did) = impl_did.as_local()
                     && let item = self.tcx.hir_expect_item(impl_did)
                     && let hir::ItemKind::Impl(item) = item.kind